In the metal-forming arts there are known a variety of stretch-forming techniques in which a workpiece, usually an elongated extrusion or a sheet-form member, is formed to the profile of a forming die surface in conjunction with the application to the workpiece of tension exceeding its yield point. The tension is applied along a line coinciding with a chord of the workpiece that is deformed during the forming process.
The known stretch-forming processes typically have been applied in the forming of aluminum alloy components such as elongated extrusion sections or thin section panels like those often used for aircraft fuselage skin.
One known stretch-forming process, for example, is often referred to as stretch wrapping or stretch-wrap forming and involves the application of mechanical tension to a workpiece to thereby stretch it beyond its elastic limit. Subsequently, while the tension is maintained, the workpiece is wrapped about a form die. The underlying principle of stretch-wrap forming is that the tension applied to the workpiece causes it to undergo plastic yield and the deformation imposed by the wrapping on the form die thus results in desirable modes of plastic flow of the workpiece material whereby the formed workpiece retains the desired formed shape substantially without springback. Thus, one advantage of stretch-wrap forming is that the form die profile may closely duplicate the final desired form and need not include springback compensation. Stretch-wrap forming is particularly well suited for forming a workpiece to long sweeping curvatures of liberal radii.
Other stretch-forming techniques include moving die arrangements, in which the gripping heads are stationary and the forming die is moved perpendicularly into the workpiece. Another technique is radial draw forming, in which one gripping head and the die are mounted on a table that rotates to slowly draw the part under tension over the rotating die.
Other advantages of stretch-forming processes generally include elimination of workpiece buckling and wrinkling, work-hardening of the workpiece, and penetration of the work-hardening throughout the section thickness of the workpiece. Furthermore, the desired results are achieved with only minimal reduction in workpiece section thickness, typically not exceeding a 5% reduction.
From the above, it will be appreciated that the known stretch-forming processes, and in particular the process of stretch-wrap forming, are vastly different from conventional bending processes as typified by the following prior art: U.S. Pat. Nos. 3,105,537; 203,842; 567,518; and 3,328,996, all of which relate to conventional bending operations in which bending force is applied laterally of the axis of an elongated workpiece. For example, in U.S. Pat. No. 3,105,537, the workpiece is bent by forming thereof over a die, whereas in U.S. Pat. No. 3,328,996, the bending is performed by relative lateral movement of a pair of dies which tend to deform the workpiece in a zone of shear therebetween. Each of the above-mentioned prior art patents also discloses the use of an incompressible fluid, specifically a liquid medium, confined under pressure within the hollow workpiece during bending.
Since the bending art as above characterized contemplates no significant application of tension force to the workpiece, certainly none of great enough magnitude to approach or surpass the material yield point in metal-forming operations, the differences between the mechanics of conventional bending, and the mechanics of stretch forming, are considerable.